Convalescent sera of experimentally infected chimpanzees or monoclonal antibodies (MAbs) specific to the 90 kDa and 40 kDa proteins indicated that both proteins were present in cytadsorging, pathogenic M. pneumoniae strains PI-1428, M129, and FH, but were not present in the non-cytadsorbing, non-pathogenic strain M129-B176. Adsorption of convalescent chimpanzee sera with the avirulent strain M129-B176 did not remove or inhibit reactivity of these two proteins. Using proteolysis and hte specific MAbs, we demonstrated that the 90 and 40 kDA proteins were surface exposed. Western blot analyses using surface-bound antibodies supported these findings. Electron microscopy studies with gold-labeled MAbs showed that the 90 kDa protein is localized on the terminal tip attachment apparatus. Thus, in addition to the well established 169 kDa P1-adhesin, the immunodominant, surface exposed 90 and 40 kDa proteins might also be involved in mycoplasma attachment. These proteins represent important immunogens and should be considered as potential candidates for inclusion in an acellular M. pneumoniae vaccine.